Melt-fabricable copolymers of tetrafluoroethylene (TFE) and perfluoro(alkyl vinyl ether) (PAVE) wherein the alkyl group contains 1 to 8 carbon atoms are disclosed in U.S. Pat. No. 3,528,954 as being prepared by polymerization in a solvent medium, preferably F-113 (CCl.sub.2 FCClF.sub.2, now known as CFC-113). The only TFE/PAVE copolymer exemplified in the Examples is TFE/PPVE copolymer, PPVE being perfluoro(propyl vinyl ether).
Reissue Pat. No. 32,199 discloses these same copolymers made by solvent polymerization to have improved toughness so as to be stress crack resistant in the form of fabricated articles, this stress crack resistance being tested by MIT Flex Life. Again TFE/PPVE copolymer is the only TFE/PAVE copolymer in the Examples. This patent discloses the rearrangement of the PAVE comonomer during polymerization to terminate the polymer chain with unstable-COF end groups. The longer the polymerization reaction, as required as PAVE comonomer content increases, the greater the time for rearrangement and adverse effect on molecular weight and copolymer properties.
U.S. Pat. No. 3,635,926 discloses further improvement in toughness of TFE/PAVE copolymer. This patent discloses that the polymer properties are poorer when the copolymer is prepared in an aqueous medium instead of a solvent medium (column 2, lines 40-48). This patent uses a buffered aqueous polymerization medium which contains certain chain transfer agents to obtain the improved result. In the Examples, Table I, F-113 solvent is shown as also being present to increase polymerization rate. F-113 is also present consistent with the belief of the need for solvent to obtain good polymer properties, notably toughness, from the earlier experience with solvent-based polymerization and the poor result obtained when water was substituted for the solvent. Example II, which does not have F-113 present, is called a baseline experiment for comparison with other runs when F-113 and the desired chain transfer agent are used. In this Table, both TFE/PPVE and TFE/PEVE copolymers are disclosed, PEVE being perfluoro(ethyl vinyl ether), with no apparent distinction between them except that where comparison can be made, the PEVE content is less than the PPVE content. PEVE contents of 1.9 wt % to 2.3 wt % are disclosed.
Japanese Patent Publication 7(1995)-126329, published May 16, 1995, corresponding to U.S. Pat. No. 5,461,129, granted Oct. 24, 1995, discloses TFE/PEVE copolymers containing 1 to 10 wt % of PEVE and having a melt viscosity of 30.times.10.sup.4 to 200.times.10.sup.4 poise (30.times.10.sup.3 to 200.times.10.sup.3 Pa.s) in order for film or sheet which is hot pressed from the copolymer to have good resistance to flex fatigue (flex life). The copolymer is made by suspension polymerization in the organic phase of a water/organic solvent medium, i.e. in the absence of dispersing agent and water-soluble initiator, and the maximum PEVE content actually obtained was 6.92 wt %.
TFE/PPVE copolymer has been the only partially crystalline TFE/PAVE copolymer which has been commercialized. The commercialization has been carried out for about the past 25 years and the product, called PFA fluoropolymer, is now available from several suppliers. This copolymer has many utilities arising from its melt-fabricability, inertness, and high use temperatures. This copolymer also has limitations on utility arising from difficulty in obtaining both high comonomer content and high molecular weight as a result of the rearrangement of the PPVE comonomer causing chain termination. This has made it difficult to serve the injection molding application to produce molded articles having high stress crack resistance; the high amount of PPVE needed for stress crack resistance gives reduced molecular weight which detracts from stress crack resistance. Even at lower PPVE contents, extrusion rates are limited because of the considerable melt viscosity of the copolymer required for copolymer toughness. This limit arises from the torque that can be delivered by the extruder or from melt fracture of the extrudate,
There has been a long felt need for TFE copolymer of similar high use temperature and inertness, but with improved melt fabrication properties, especially for extrusion, including injection molding, which are the major fabrication processes used, and without the limitations on composition and molecular weight which have limited the use of TFE/PPVE copolymer in the past. There is also a need, for environmental reasons, to be able to make the copolymer without the use of organic solvent in the polymerization medium.
Aomi et al. in U.S. Pat. No. 5,064,474 disclose the problem of fine particles of submicron size in molded products of fluorine-containing resin, for example, containers for reagents used in semiconductor processing. Such particles contaminate the reagents and are deleterious to production of defect-free semiconductor devices. The '464 patent, to solve the problem, offers a multiplicity of methods for removing the fine particles from articles. A fluoropolymer resin having a substantially reduced fine particle component is desired.